The principles behind luminance of light emitting diodes are by injecting an electric current sequentially through P-N junctions of a semiconductor. The material of AlGaInP is implemented for high brightness red, orange, yellow and yellowish green LEDs and AlGaInN is for blue and green LEDs. The process of metal organic vapor phase epitaxy (MOVPE) is commonly adopted in the mass production of the LEDs, while the light-emitting components are of the structures, including: homo-junction (HOMO), single-heterostructure (SH), double-heterostructure (DH), single-quantum well (SQW) and multiple-quantum well (MQW) or other appropriate structures.
The structure of a conventional light emitting diode is illustrated in FIG. 1A, including, from a top thereof down formed with a front electrical electrode 11, a transparent oxide layer or a window layer 14 to disperse the current, an active layer 12, a substrate 10 and a back contact 13. Among them, the active layer 12 is formed by a light-emitting material, such as AlGaInP or AlGaInN by adopting MOVPE and the transparent conductive oxide layer 14 is a transparent conductive oxide layer typically formed of an indium tin oxide (ITO) layer. After a current is injected through the front contact 11, the current will pass through the transparent conductive oxide layer 14 to disperse and then through the active layer 12 and the substrate 10 to flow towards the back contact 13. Light is emitted when the current flows through the active layer 12. The active layer 12 is a sandwich layer, including a p-type upper cladding layer, an intrinsic layer and an n-type lower cladding layer. However, the low carrier mobility and high resistance of the active layer made of AlGaInP or AlGaInN results in poor electric conductivity of the AlGaInP or AlGaInN. Apart from that, the transparent conductive oxide layer 14 though can improve the current dispersion; its conductive ability is, however, weaker than that of a metal layer. Consequently, a metal grid layer (not shown) is generally embedded in the ITO layer 14, e.g., a metal grid layer formed on the active layer, and then covered it by forming an ITO layer thereover. The current distribution is getting improvement, nevertheless, the primary emitting regions are mainly concentrated at and next to the lower portion of the electrode, as shown in FIG. 1A.
The forgoing processes are exemplary for a light emiting semiconductor. However, due to auniformity manufacturing processes, the light color emitted from a LED die taken from a position A of a wafer is generally found to be deviated from that of another at a position distant from the position A. In practice, as long as a size of a wafer is over 2 inches in diameter, the wavelength emitting from the die 30 at the center portion of the wafer is found different from those of the dies 32, 33, 34 located from at the edges of the wafer. For instance, for a blue light LED wafer is concerned, light with a central wavelength emitted by a die 30 is assumed to be 440 nm but the variation of the central wavelength of the dies 34 may have light with a central wavelength 30-40 nm offset. The color deviation result may be further worse for two dies taken from the same relative position but different batches . . . . Consequently, it has been troublesome to the industry to discard those unqualified LEDs or not just due to a central wavelength out of a criteria range.
To LED illuminating industry, a small central wavelength shifting is generally acceptable but it doesn't if the LEDs are to mix with others so as to produce a desired decorative pattern. For example the appliance such as TV, notebook, and monitor using LED backlight, the quality criteria is stricter, particularly, to those high-end models.